A membrane refers to a porous physical barrier which allows a specific component in a multi-component mixture to be selectively passed therethrough, thereby separating components of the mixture. Advantageously, water treatment using a membrane requires less chemicals such as coagulants than other filtration processes and allows reduction of required installation space. A membrane for water treatment can separate organic pollutants, inorganic pollutants, and microorganisms in water depending upon the kind thereof. Pressure-driven membrane are divided into a microfiltration (MF) membrane, an ultrafiltration (UF) membrane, a nano-filtration (NF) membrane, a reverse osmosis (RO) membrane, and the like according to pore size. In addition, membranes are also divided into a polymer organic membrane and an inorganic membrane of ceramics or metals. Examples of a membrane module (or film module), which has a membrane disposed in a housing and configured to perform solid-liquid separation, include tubular, hollow fiber, spiral wound, plate and frame, and rotary disk membrane modules.
Such a membrane has innumerable fine holes, i.e. membrane pores. Thus, pollutants contained in raw water are accumulated on a surface of the membrane, or adhere to the surface of the membrane or an inner side of the pores when filtered out by the membrane or passed through the membrane, thereby causing contamination of the membrane. When the membrane is contaminated, the membrane is covered with accumulated pollutants, or the membrane pores are reduced or plugged during filtration, thereby causing deterioration in raw water permeability and performance of the membrane, and thus periodic membrane cleaning is performed during filtration in order to recover performance of the membrane.
A method for cleaning a membrane may be divided into physical cleaning and chemical cleaning. Typical examples of physical cleaning are back-pulsing using water and aeration using air. Back-pulsing is a process wherein treated water is pushed in a direction opposite to a filtration direction to pass through a membrane, thereby removing materials accumulated on a surface of the membrane or in membrane pores. Aeration is a process wherein air is blown to the surface of the membrane to remove accumulated pollutants.
As an air cleaning method for a submerged membrane typically used in a water treatment apparatus, cyclic aeration and automatic aeration filtration (AAF) are widely known in the art. Such conventional methods were developed to minimize membrane contamination and increase energy efficiency. However, cyclic aeration has a disadvantage of exhibiting poor efficiency under raw water conditions of severe variation in water quality. On the other hand, AAF is an air cleaning method wherein change in aeration method and adjustment of aeration intensity are achieved based on turbidity of membrane inflow water and membrane contamination index, and can thus cope with variation in quality of raw water and can be associated with automatic control. However, AAF has a disadvantage in that optimum aeration intensity equation for the membrane contamination index must be derived again when the water system of inflow raw water is changed.